1. Field of the Invention
This invention relates to a control system for a vehicle automatic transmission, and more particularly to a control system for a vehicle automatic transmission which always determines the gear ratio optimally during traveling a place at any altitudes.
2. Description of the Prior Art
There have been proposed various vehicle automatic transmission control systems. One example of such a system is described in Japanese Patent Publication No. Sho 59-8698. The assignee proposed a similar system in Japanese Laid-Open Patent Publication No. Hei 5-71626, which was also filed in the United States and matured to Patent (U.S. Pat. No. 5,317,937). In the system there are prepared in advance separate gear shift scheduling maps (gear shift diagrams or programs) for different road profiles--one for level-road driving, another for up-hill driving, and so on, and a parameter indicative of driving resistance such as vehicle acceleration is obtained. Then it is estimated on the basis of the parameter if the vehicle is hill climbing or hill descending and based on the determination, one map is selected among them and a gear ratio is determined in accordance with the selected map. More specifically, the vehicle acceleration is predicted from the vehicle operating conditions including engine output torque and is compared with the actual vehicle acceleration to estimate the degree of hill climbing/descent, i.e, the road profile on which the vehicle is traveling, on the basis of which one among maps most suitable for the traveling condition is selected and a gear ratio is determined on the basis of the selected map.
In these prior art systems, however, the predicted vehicle acceleration is usually determined in advance at a relatively low land on the basis of the vehicle operating conditions including engine output torque and is stored in a computer memory mounted on the vehicle. In other words, no attention is paid to the change in engine output torque at high lands or altitudes. Therefore, when retrieving the predicted vehicle acceleration from the memory and comparing it with the actual vehicle acceleration to estimate the road profile on which the vehicle is traveling, since the engine output torque decreases at altitudes due the charging efficiency drop, there is a possibility that the road profile is estimated steeper than it really is.
It is therefore an object of the invention to provide a control system for a vehicle automatic transmission by obtaining a parameter indicative of the driving resistance to estimate hill climbing or hill descending in order to select one among a plurality of gear shift scheduling maps and then based on the selected map to determine a gear ratio to be shifted to, which overcomes the aforesaid drawbacks of the prior art systems and which is able to correct the parameter indicative of the driving resistance in response to the altitude of a place where the vehicle is traveling such that the gear ratio can optimally be determined no matter where the vehicle is traveling.
Further, the aforesaid drawbacks will be serious as the altitude of a place where the vehicle is traveling increases. Therefore, in views of the drivability, the parameter correction should preferably be conducted at an early time. More specifically, the parameter correction should preferably be sped up with increasing altitude.
It is therefore another object of the invention to provide a control system for a vehicle automatic transmission by obtaining a parameter indicative of the driving resistance to estimate hill climbing or hill descending in order to select one among a plurality of gear shift scheduling maps and then based on the selected map to determine a gear ratio to be shifted to, which overcomes the aforesaid drawbacks of the prior art systems and is able to correct the parameter indicative of the driving resistance in response to the altitude of a place where the vehicle is traveling such that the gear ratio can be optimally determined no matter where the vehicle is traveling and the parameter correction is sped up with the increasing altitude, whereby enhancing the drivability.